1. Field of Invention
The invention concerns a blow head for a multilayer film. Blow heads are known. Blown films are extruded with them in blown film extruders.
2. Description of the Prior Art
For this purpose, one or more extruders convert a plastic extrudate to a plastic melt, which is fed to the blow head. For this purpose, melting lines are used, which connect the extruder or extruders to the blow head. Generally, the corresponding melt lines continue in the blow head. However, here they discharge in gaps within the blow head. In blow heads for production of multilayer film, several such gaps are provided in each blow head to guide a single stream. The individual layers in these gaps are actually formed. For this purpose, the individual layer or individual stream is converted from the cylindrical shape in the feed line to a flat shape in wind-up. In high-grade blow heads, the conversion can be conducted by guiding the melt line in the peripheral direction (φ) of the blow head or the film tube and then grading it into the flat single gap. The feed line can then grade as a coil or sleeve into the gap. The zone, in which this transition occurs, is called the melt distribution zone in this document.
Often, but not always, a flow zone is connected to this zone, in which the melt flows along the gap for guiding an individual stream and, in this case, is even more strongly “accustomed” to the new flat shape. Blow heads are also known, in which a feed line grades directly into a flow zone without an extra pronounced melt distribution zone. Finally, the gaps to guide an individual stream in blow heads for production of multilayer films discharge into a gap for combining the film composite, which is ultimately designed as an annular gap. The different film layers adhere to each other in this annular gap. The annular gap ends on the round extrusion nozzle, at which the melt tube leaves the blow head.
It is also known from DE 10 2004 001 473 A1 to measure and control the thickness of the entire film in such blow heads. For this purpose, electrically operable cartridge-type heaters are arranged in the vicinity of the annular gap. These are in thermal contact with the melt and are capable of supplying heat selectively to sectors of the peripheral surface of the melt tube.
They are activated when a measurement device that measures the film thickness on the periphery of the extruded film tube finds a thick site in such a sector of the peripheral surface. Heating of the melt of the corresponding sector increases its stretchability, so that the corresponding sector is stretched out more strongly by the internal pressure in the bubble than the untreated sectors. Stronger stretching out eliminates the thick site.
It is known from EP 0 435 786 A2 to influence the temperature of individual melt layers overall by massive cooling. In this way, it is supposed to be possible to connect individual layers with strongly different processing temperature relatively damage-free into multilayered film by cooling the hotter processed melt layer before it is supplied to the more heat sensitive film.
Multilayer films have different film layers from sometimes high-grade film material.